Radio control system



Patented ug. 30, 194@ RADIO CONTROL SYSTEM Gilbert G. Brown, Grange, and George J. Eannarino, Arlington, N. J., assignors to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application November 30, 1946, Serial No. 7213,244

7 Claims. 1

The present invention relates to radio control systems and more particularly to remote control systems wherein the operation of radio receivers may be controlled by transmitted radio signals.

Communication systems in which transmissions from a main station must be directed to selected receivers of a plurality of associated receivers are extensively employed by such organizations as police departments, many of which at the present time utilize for the receiving equipment a plurality of mobile receivers provided with so-called squelch circuits` Each of these circuits theoretically functions to mute the associated audio system of a receiver except at such times as a desired carrier reaches the receiver. One of the greatest drawbacks to presently known mobile receivers of this type, however, is the fact that, when a F. M. transmitter from a station overmodulates and therefore switches its carrier into an adjacent channel, a receiver tuned to the adjacent channel frequency may reproduce the signals intended for another receiver.

It is therefore one of the main objects of the present invention to provide a radio control system wherein muting devices for the radio receivers are utilized which avoid the above-mentioned difficulty, and another object is to provide a control system wherein receivers may be made selectively operable by the transmission of an audio note and then maintained operative by the carrier.

With these and other objects in view, the present invention will be explained in the following detailed description which is to be read in conjunction with the block and schematic diagrams shown in the accompanying drawing.

The radio control system as an integrated unit comprises at least one transmitting system I and preferably a plurality of receivers, 2 and 3 being representative. Transmitting system I is adapted to radiate at least one carrier, and this carrier may be either amplitude or frequency modulated. depending upon preference and system requirements, by the desired intelligence signals. Additionally, the transmitter 4 may be modulated by any one or a combination of a plurality of audio signals from the source of reed.. frequency signals 5, these signals being selectively coupled to the transmitter, for purposes which will later become obvious, by means of switches Ei, l etc.

A receiver arrangement suitable for use in the system of this invention is disclosed in block form in the lineup designated generally by numeral 2. The high frequency stages B deliver' their output to a detector =9 which is in turn coupled to audio amplifiers Ill, and the audio reproduction is accomplished by speaker I I which concludes the conventional receiver lineup. Muting of the receiver is achieved by a switching system I2 shown coupled to the audio ampliers lil. Proper excitation for the switching system I2 may be derived from'the high frequency stages 8 and also from a tuned reed system I3. The system I3 receives its actuating signals from the audio ampliers I0.

Fundamental operation of the radio control system may be outlined as follows with respect to the preceding disclosure. Transmitting system' l radiates intelligence signals modulated upon a certain carrier, which carrier may in turn be picked up byreceiver 2, amplied in stages B and detected by 9, the audio components Ibeing amplified and reproduced by amplifiers I0 and speaker Il. 'Switching system I2 is adapted to prevent transmission of signals to the speaker 4from amplifiers I0 except when said switching system is actuated by certain impulses or electrical conditions from stages S or reed system I3; If it is desired that the receiver be made operable to deliver an audible output simply upon reception of a desired carrier, switch I4 should be closed, thus coupling stages 8 with the switching system I2 and supplying the system with said certain impulses when the desired carrier is received. These certain impulses, which are delivered by stages 8 only when the desired carrier is received, also appear in coupling line I5 between stages 8 and system I2, although the switching system I2 is adapted to be actuated by these impulses only when such impulses have rst'been received either from stages 8 through switch Ill, or, as later described, from reed system`l3.

If, however; it is preferred to render the receiver operable only when a certain tone modulates the transmitted carrier, switch, I4 left open. Transmitter 4 may be modulated by said certain tone from the source of reed-irequency signals 5 when one or another or a combination ofswitches 6, l etc. are closed. At receiver 2, said certain tonev will be delivered to tuned reed system I3 by amplifiers l, and the system I3 will thereupon transmit said certain impulses to the input of switching system I2 which will operate the receiver. Once the switchingA system I2 has thus been actuated by impulses from reed system I3, impulses from stages 8 delivered by coupling line I5 will maintain actuation thereof while the desired carrier is received, even though said certain tone is no longer modulated on the carrier. Cessation of the carrier operates to stop actuation of switching system I2 and the receiver thereupon becomes inoperative until the tone is again received.

The diagram of the receiver designated by numeral 3 illustrates in greater detail the embodiment of the present invention. This receiver, which may be identical with the receiver 2, is designed for reception of frequency modulated signals, and, included in the receiver lineup are R. F. amplier I6, local oscillator I1, mixer I8, I. F. amplier I9, thekrst limiter stage 20, the second limiter 2|, discriminator '22 and audio amplifier stage 23, a power amplifier stage 24 and a speaker 25. That portion of receiver 3 which corresponds to the switching system I2 in receiver 2 includes primarily the double triode 26 and the multiple contact relay 21; and the tuned reed system I3 of receiver 2 has as its counterpart in receiver 3 the tuned reed switching arrangement 28 utilized in conjunction with the battery 23.

The receiving set 3 as shown in the drawing can as previously described with respect to receiver 2, be used either for carrier controlled operation or for selective calling operation wherein c, selected signal emitted by the transmitter controls the reproduction of intelligence by the receiver.

Referring now to the carrier controlled operation of receiver 3, one portion of the control circuit includes a D.C. amplifier as a triode 30, and a relay control tube as a triode 3|, with the associated relay 21. Both triodes 30 and 3I are preferably in a single envelope. As also disclosed in the drawing, the grid voltage of the rst limiter tube 32 may be coupled to'bias the triode 30 negatively through a switch 33. The switch 33 is closed when the circuit is to be adapted for carrier controlled operation. The normal no signal control grid voltage of the rst limiter tube 32 is used to set the bias of the triode 30 to approximately zero, which permits the triode 30 to conduct. The flow of current through the resistance 34 which is connected with B+ results in a considerable voltage drop and establishes a low value of plate voltage on the plate of the triode 35. It further determines the voltage at the junction of the resistances 35 and 36 which connect resistance 31 with the grid of triode 3l, which junction voltage is also low and of positive polarity.

The grid of the triode 3l' is connected through the resistance 31 to the mentioned junction of the resistance 35 and 36 and is also connected through the resistance 38 to B-.

In the normal no signal condition the positive voltage across the resistance 36 and the negative voltage across the resistance 31 add up algebraically and result in a negative voltage on the grid of the triode 3| which is greater than necessary for the cut-off of the plate current. The winding of relay 21 in the plate circuit of the triode 3I therefore remains unoperated.

Y The small amount of current flowing through the winding of relay 21 due to the resistances 39 and 40 and 4I connecting the winding of relay 21 with ground is not suicient to operate the relay 21. Since the relay 21 is not vsufficiently sensitive to operate reliably on the value of the plate current of triode `3I, when it is conducting, due to the low voltage supply for the plate, resistance 4I is used to pass additional 'current 4 through the winding cf relay 21 and provide positive operation.

Positive voltage is also applied to the grid circuit of the triode 35 through the winding of relay 21, resistance 4I), resistance 42, resistance 43 and resistance 44 which is grounded. The voltage at the grid of the triode 30 is the algebraic sum of the voltages across the resistance 44, across the part of the potentiometer 43, that is, between its arm and the resistance 44, and across the bias cell 45. With no signal the Voltage between the grid and the cathode of the triode 3@ is approximately zero.

When now a carrier is received, the grid voltage of the rst limiter tube 32 increases. This increases automatically the negative voltage across the resistance 44, since the switch 33 is closed and, therefore, also between the grid and the cathode of the triode 30. A carrier signal of three-tenths of a microvolt at receiver input terminals will cause the negative grid voltage of the rst limiter tube, and therefore also the negative grid voltage of the triode 33 to increase suilciently to result in a plate current cutoff of the triode 30.

If no current flows through the triode 30, the voltage drop across the resistance 34 decreases considerably resulting in a much higher plate voltage. rlhe net result of these actions is to produce a greater positive voltage drop across the resistance 36 and, therefore, a decrease of the negative voltage between the grid and the cathode of the triode 3i to approximately Zero. The triode 3l will then conduct and the plate current of the triode 3l will operate the relay 21.

Upon operation of the relay such as by reception of a carrier for which the receiver is tuned the following changes in the circuit will take place:

1. The contacts 46 and 41 will be closed, contact 46 being connected to A-lwhereas contact 41 is connected through the cathode of output tube 48 to ground so that the closing of the contacts 46 and 41 causes energization of the cathode of output tube 43.

2. Since the coil 4S of the vibrating reed 28 is, before operation of the relay 21, connected with the plate circuit of the rst audio tube 50 by means of the connected contacts 5I and 52, operation of the relay 21 disconnects contacts 5I and 52 thereby lremoving the Vibrating reed coil '49 from the plate circuit of the rst audio tube.

3, The opening of the normally connected contacts 5I and 52 brings about a connection between the contacts and 53, Contact 52 being connected with the plate circuit of the rst audio tube 55 and the contact 53 being connected with the grid circuit of the output tube 48, thereby connecting the plate circuit of the first audio tube 5i) with the grid circuit of the output tube 48.

4. The contacts 54 and 55 are also closed upon operation of the relay 21 thereby completing a second circuit from the grid of the first limiter tube to the grid of the triode 3G. However, since the switch 33 secured this connection before between the two mentioned grids no change will take Vplace upon closing of the contacts 54 and 55 in the application of the disclosed circuit for carrier controlled operations.

When the carrier is terminated, the grid voltage `of the first limiter tube decreases to its normal value. This decreases the negative voltage between the grid and the cathode of the triode 30 to approximately zero. Consequently the action as described previously for no signa condition will follow, the relay 21 becomes deenergized and the receiver cuts off. The set is then in condition ready to be turned on again by reception of a carrier.

It is important to emphasize that the receiver in connection with the car call system will respond only to a carrier of frequency to which the receiver is tuned.

Whereas above the application of the circuit for carrier controlled operation has been described, the same circuit will now be described for selective calling operation. For this use a vibrating reed 28, which will respond to only one audio frequency, is used with the circuit as described above.

, It is to be noted that the switch 33 arranged between the grid circuits of the first limiter tube and the triode 30 will be open. Consequently in the normal no-signal condition, there will be no connection between the grid circuit of the first limiter tube and the grid circuit of the triode 30, therefore, no negative voltage from this source is applied to the grid circuit of the triode 3|), the grid voltage will be approximately zero and the triode 3|] will conduct. The winding of the relay 21 in the plate circuit of the triode 3| will not operate for the same reason as disclosed above in describing the carrier controlled operation.

In the case where a carrier is received, the grid voltage of the first limiter tube will increase, however, since there is no connection between the first limiter tube and the triode 3U no change will take place in the circuits of the triodes 30 and 3| and the relay 21 will not operate.

If then an audio tone of the frequency to which the vibrating reed unit 28 will respond is impressed on the carrier, amplification by the first audio tube 50 from the discriminator output will take place and will actuate, through contacts 52, the reed of the vibrating reed unit 28.

When the reed has attained sufficient amplitude the movable contact 56 will close contact with the xed contacts 51 thereby completing momentarily a circuit from the biasing voltage source to the Ygrid circuit of the triode 30. Negative bias will be applied then to the grid of the triode 30. By this action the plate current of the triode 3B will be cut olf and the relay 21 will be operated by means of the plate current of the triode 3| in the same manner as described above in connection with carrier controlled operation. This negative biasing voltage may, of course, be obtained from sources other than a biasing battery, for example, from the grid circuit of the second limiter tube where a high negative bias always appears, even when no carrier is received because of thermal noise amplication in preceding stages. The operation of the relay 21 produces closing of the contacts 46 and 41, opening of the normally closed contacts 5| and 52 and closing at the same time of the contacts 52 and 53 with the same result as disclosed above in connection with the carrier controlled operation. However, the closing of the contacts 54 and 55 completes now for the first time a circuit from the grid of the first limiter tube to the grid of the triode 30. This arrangement permits the output of the receiver to remain on even after the audio tone is removed. The relay 21 will remain operated and the output of the receiver will remain on as long as a car- 6e rierls being received and will go off only when the carrier is no longer received.

In order to provide the operation of the relay 21 by hand, when an audio tone might not be received, the winding of relay 21, which is on one side connected with B, is connected on its other side over a resistance 51 and a hand operated switch 58 with ground. Upon closing the switch 58 by hand the relay 21 will be put in operation, regardless of whether or not the plate circuit of triode 3| provides sufficient current, and will remain in operation until the switch 58 is opened again.

4 When the carrier drops out, the grid voltage of the first limiter tube 32 decreases again to its normal value. This will consequently decrease the negative voltage between the grid and the cathode of the triode 30 to approximately Zero. The action, as described above for no slgnal condition, will follow, the relay 21 will be de-energized and the receiver will go off. It is now ready to be turned on again by reception of the carrier and of an audio tone.

The above described circuit and the resistance value are so arranged in proportion that a positive triggering action is obtained. This is achieved in the following manner: As the triode 3| starts to conduct, the current through the relay Yresistance causes a voltage drop. As soon as a small amount of this voltage is introduced into the grid circuit of the triode 3l), to set up the original no signal condition, any decrease in the positive voltage will tend to increase the negative vvoltage at the grid of the triode 3i), which tends to increase the plate current of the triode 3|. This action is cumulative and continuous until a maximum value of the plate current flows in the triode 3| and no additional plate current flows in the triode 30. Any further increase in the first limiter grid current has no eifect upon the operation.

Proper proportioning of the resistance values and the use of the bias cell results in a sensitivity of operation that is substantially uniform with normally encountered changes of battery voltage and plate supply voltage.

The sensitivity of the set is adjustable by varyingthe potentiometer 43. If the arm of 43 is moving in a direction to increase a resistance between the arm of potentiometer 43 and the resistance 44, a greater positive voltage in the grid circuit of the triode 30 will result. Therefore, a greater negative voltage in the grid circuit of the first limiter tube is required to cause the desired actions to operate the relay 21. The grid voltage of the first limiter tube 32 is a function of the signal strength, and up to a certain point increases with the signal strength. However, further increase of the signal strength beyond that value does not cause any further increase in the grid voltage.

It is not essential to the operation of the present invention that there be filament control of any tube. However, in mobile receivers the filament power consumption during periods when no signal is received is an important factor, hence filament control of the output tube of receiver 3 has been illustrated.

The system described above has the particular advantage that a certain predetermined audio tone alone is able to put the set in operation, and, once thus operated, the set will remain in operation after removal of the audio tone as long as the carrier is received and will cease operation only upon removalof the carrier.

While the principles of the. invention havebeen disclosed in detail with respect toa mobile frer quency modulation receiver, i-tis clear that the same principles: can be usedVE in connection with an amplitude modulation receiver by adapting the receiver to amplitude modulations as is Well known tomenskilled in the art. The system as a` whole' has been diagrammed with only a single transmitter and two receivers, although, of cou-rse', any number of transmitters and' receivers may be ini-- corporated therein. Also, each receiver' may be adapted= to respondJ toa tone which is characteristic to its own reed alone, or, several receivers may be adapted to respond' to the same tone. Any required number of tones may be derived from the source of reed frequency signals 5 and these may be impressed individually or' simultaneously upon the transmitter to cause appropriate modulation of the carrier, depending upon whether it is desired to cause one or a plurality of receivers torespond to thel transmitter signals. It may also be desirable to maintain operation of any receiver only as long as a certain characteristic audiok tone is received, in which case this tone may be blocked from reaching the speaker and then with certain obvious changes the system diagrammed may function suitably,

While we have disclosed the principles of our invention in connection with preferred embodiments, it should be understood that these embodiments are given by way of example only and not as limiting the scope of the invention as set forth in the objects and the appended claims.

We claim:

l. In a radio communications system wherein receivers are responsive to certain characteristic audio frequency tones transmitted from a source of frequency modulated signals, a lfrequency modulation receiver system including high frequency stages at least one of which is a limiter stage, demodulator means, audio amplier stages, and intelligence signal reproducing means, a multiple contact relay switching means', said relay switching means including an operating coil, twol Variable impedance devices, and a power source, the second of said variable impedance devices being connected in series with said operating coilY and said power source, the second of said impedance devices comprising a switching means for actuating said relay switching means, and the first of said impedance devices comprising a vacuum tube amplifier biased by energizing voltages to vary the impedance thereof and control said relay switching means, one pair of normally open contacts of said relay switch-ingmeans beingv cont nected in the signal path of said audio amplifiers, a vibrating reed switching system responsive to a characteristic audio frequency signal and coupledv with an output of said audio' amplifiers through a pair of normally closedcontacts of said relay means, an energizing voltage source coupled to said vacuum tube amplifier through con-tacts of saidI vibrating reed switching means., and another pair of normallyv open contacts of said relay means being connected between said vacuum tube amplifier andE a point in said high frequency stages where a certain excitation voltage occurs only while saidv frequency modulating signals are being received.

2; In a radioA communications system asset forth in claim 1, said relay switch-ing means comprising a first vacuum tube 'cia-sed to cond-uct only when said energizing voltages are not. coupled thereto, and a second vacuum tube connected in series with said operating coil and said power 2lil d swrce and coupled tor said iirs-t vacuum. tube such that the impedance thereof decreases when said first'- vacuurn tube is rendered non-conducting. by said energizing voltages.

tl. In a radio communications system as set forthi inv claim I, said point in said highl frequency stages being a connection to the control grid of a rst limiter' stage in the high frequency stages of said frequency modulation receiver system.

41 In a. yradio receiver, a source of received carrier energy, a detected signal source, audiov amplifier means connected to said detected signal source, a tuned reed having a pair of normally open contacts, a normally de-energized power source, arelay havin-g an operating coil, three pairs of normally open contacts and a pair of normally' closed contacts, an operating coil for said tuned reed, means normally connecting the opera-ting coil of said tuned reed toy said detected1 signal source through the normally closedv con-v tacts of' said relay, whereby said tuned reed operates to close its con-tacts upon receipt of an audio signal of'x predetermined frequency, means for energizing said normally cie-energized power sourceV through said tunedV reed contacts whereby the operating coil of said relay is energized' and wherebyv the normally open contacts of said relay will close and the normally closed contacts Will open, means for energizing said audio amplifier means through one pair of normally open contacts of said relay, means for' energizing said normally de-energized power source from said carrier source through a second pair of normally open relay contacts, and substantially simultaneousl'y opening the energizing circuit for said tuned reed coil through said normal-ly closed contacts and connecting said detected signal source to said audio amplifier means through the said third pair of normally open contacts, whereby said source. of' received carrier energy maintains the said. relay ln operation after removing the said signal of predetermined. frequency.

5. In a radio receiver, as set forth in claim 4, said other amplifier means comprising` a direct current amplifier.

6.. In a radio receiver, a source of received carrier energy, a detected signal source, audio amplifier means connected to said detected signal source, a tuned reed having a pair of normally open. contacts, other ampl-ier means, a relay having an operating coil, threey pairs of normally open contacts and a pair of normally closed contacts, an operating coil for said tuned reed, means normally connecting the operating coil of. said tuned. reed to said detected signal source through the normally closed contacts o-fk said relay, whereby said. tuned reed operates toclose its contacts upon. receipt of an aud-io signal of predetermined frequency, means for energizing said other amplifier means through said tuned reed contacts whereby the operating coil of sai-d relay is energized and the normally open contacts of said relay will close andthe normally closed contacts will open, means forenergizing said audio amplier means through onev pair of normally open contacts of said relay, means for energizing said? other amplifier means from' said carrie-r source through second pai-r of normally open relay contacts, and substantially simultaneously opening the energizing circuit for said tuned reed coil throughsaid normally closed contacts and connecting said detected signal source to said audio amplifier means through the said' third pair ot normally openA contacts, whereby said source oli received carrier energy maintains the said relay in operation after removing the said signal of predetermined frequency.

7. In a, frequency modulation radio receiver of the type having a first and a second limiter, and a discriminator, an audio ampliiier connected to said discriminator, a tuned reed having a pair of normally open contacts, a direct current amplier, a relay having an operating coil, three pairs of normally open contacts and a pair of normally closed contacts, an operating coil for said tuned reed, means normally connecting the operating coil of said tuned reed to said discriminator through the normally closed contacts of said relay, whereby said tuned reed operates to close its contacts upon receipt of an audio signal of predetermined frequency, means energizing said direct current amplifier through said tuned reed contacts whereby said operating coil of said relay is energized, means for energizing said audio amplier means through one pair of normally open contacts of said relay, means energizing said direct current ampliiier from said iirst limiter through a second pair of said normally open relay contacts, and substantially simultaneously opening the energizing circuit for said tuned reed coil through said normally closed contacts and connecting said discriminator to said audio amplifier means through the said third pair of normally open contacts, thereby disconnecting said direct current amplifier from said second limiter and l,connecting said direct current amplifier With said first limiter, the latter maintaining said relay in operation after removing the said signal of predetermined frequency.

GILBERT G. BROWN.

GEORGE J. EANNARINO.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,608,969 Webbe Nov. 20, 1926 2,245,652 Dickert June 17, 1941 2,321,651 Caraway June 15, 1943 2,325,829 Boswau Aug. 3, 1943 2,369,230 Hansell Feb. 13, 1945 2,392,672 Koch Jan. 8, 1946 

